Currently, a dry cleaning process that uses an ultraviolet beam is known, for example, as a cleaning process used in the optical ashing treatment to be applied to a resist during a semiconductor element manufacturing process, a liquid crystal element manufacturing process or the like, a dry cleaning process used in the removal treatment to be applied to a resist that adheres to the pattern surface of the template of the nanoimprinting device, a dry cleaning treatment to be applied to a glass substrate of a liquid crystal element, a silicon wafer or the like, and a cleaning process used in a removal treatment (desmearing treatment) to be applied to a smear created during a printed circuit board manufacturing process. In particular, a dry cleaning process that uses ozone and reactive (radical) oxygen created with a vacuum ultraviolet beam emitted from an excimer lamp is often employed because it can finish a desired processing in a short time in an efficient manner. Different types of such light irradiation devices are proposed to date (see, for example, Patent Literature Documents 1 to 3).
One type of the light irradiation device that uses the vacuum ultraviolet beam is, for example, shown in FIG. 3. Beams from a plurality of ultraviolet lamps 41 that emit vacuum ultraviolet beams are directed, via a light transmitting window member 12, to a target surface (surface to be processed) Wa of a workpiece W placed in a processing gas atmosphere such as an oxygen gas atmosphere.
In this light irradiation device, the ultraviolet lamps 41 constitute a light source unit 40. The light source unit 40 has a casing 42 that has a box shape with an opening in one face thereof (lower surface in FIG. 3). A flat plate-like light transmitting window member 12 is disposed at the opening of the casing 42 to air tightly close the opening. In the casing 42, there are provided a plurality of rod-shaped ultraviolet lamps 41 such that the center axes of the lamps extend in parallel to each other in the same horizontal plane. A reflection mirror 43 is disposed to surround the ultraviolet lamps 41.
The workpiece W is placed on a workpiece placement surface (loading surface) 31a of a workpiece support (supporting table or stand) 31 of the light irradiation device. The workpiece placement surface 31a has a greater size than the target surface (surface to be processed) Wa of the workpiece W in the length direction and the breadth direction. The workpiece support 31 has a heating unit (not shown) for heating the workpiece W. The workpiece support 31 has a through hole 32a for supplying a gas, and another through hole 32a for discharging the gas. Each of the through holes 32a and 32b has an upper end that is open to the workpiece placement surface 31a. The through holes 32a and 32b are spaced from each other in the plane direction (the direction in which the lamps are arranged) such that the workpiece W is placed between the upper end of the through hole 32a and the upper end of the through hole 32b. 
In the illustrated example, the reference numeral 34 designates one of two column-shaped spacer members that form a space having a predetermined volume between the light transmitting window member 12 and the workpiece support 31. A sealing member 35 is disposed on top of each spacer member 34. The light source unit 40 is air tightly disposed on the workpiece support 31 with the sealing members 35 therebetween. Thus, a processing chamber (treatment chamber) S is defined between the light source unit 40 and the workpiece support 31.
In FIG. 3, the gas flowing direction is indicated by arrows.